The present divisional application relates to a method for replacing objective parts, especially of a projection or illumination objective for microlithography.
Modern lenses in lithography increasingly have replaceable or exchangeable elements, such as replaceable or exchangeable lenses, filters, diaphragms and the like. US 2006/0176460 A1 describes, for example, an EUV (extreme ultraviolet) lithography system having optical elements, such as mirrors and the like, which are individually selectable for use, wherein the optical elements are not replaced, in the sense of being moved into and out of the objective, but rather are exchanged only in the form of a rotatable turret. However, this imposes a very high space requirement.
WO 2006/069755 A1 describes, in contrast, a replaceable optical element of an objective module of a lithography system, which can be completely removed from the objective, wherein a proposal is made to avoid contamination of the objective room by providing a load-lock chamber.
When parts located in the highly clean, gas-flushed or evacuated objective interior are replaced, there is a problem that the process of replacement can introduce impurities into the objective. In addition to contamination via the gas room during the replacement, contaminants adhering to the replaceable components can be introduced into the objective, said contaminants subsequently capable of leading to a deterioration of the objective characteristics. Possible contaminants in this context are particularly hydrocarbons, water deposits and other particles.
The hydrocarbons, which are mostly present in the form of monolayers on the surface of the replacement parts, mostly do not become detached until irradiation with the objective light used for imaging, for example, UV laser light. Accordingly, unless further cleaning takes place, this means that hydrocarbons are present in the objective and can then enter into chemical reactions at undesirable regions, and so lead to deposits on the optical elements. As a result, the imaging properties of the objective are impaired.
Similar considerations apply to monolayers of water, which are also present on replacement parts that are exposed to the ambient atmosphere. In the normally extremely dry environment within the objective, the water monolayers desorb or evaporate and are then also in the objective, as a result of which ozone, which is highly reactive, can form due to UV light. The water molecules, too, can then enter into reactions, for example, with hydrocarbons that are also present in the objective interior or with other components in the objective interior, such that, due to the monolayers of water as well, deposits can form on the surfaces of the optical elements, especially salts. To prevent imaging damage, US 2006/0001854 A1 accordingly proposes the provision of optical elements having several usable areas in the objective, such that, following contamination of one area, a different, clean area can be moved into the beam, while the contaminated area can be cleaned.
JP 11288870 A proposes exchanging contaminated areas and subsequent cleaning of the contaminated areas for a protective device between the projection objective and the wafer of a lithography system.
A further problem of replaceable components of the optical system of a lithography system is that particles adhering to the replacement parts can become detached from the replacement parts and precipitate on the surfaces of the optical elements, a fact which also impairs the imaging properties.
These problems have so far been counteracted by cleaning the replaced objective parts or the objective interior with cleaning gas for an adequate length of time. As a result, the desorbed or evaporated water monolayers or the hydrocarbons or residual parts thereof which have been transferred into the gas phase during illumination are driven out of the objective, such that the corresponding negative deposits are avoided.
However, this approach has the disadvantage that, first, the contaminants are introduced into the objective interior and there is a risk that, on account of the subsequent cleaning, said contaminants are not completely removed from the objective interior, such that, for example, a device according to US 2006/0001854 A1 is required. Furthermore, this approach means that the objective is inoperable for a long period because, during this time, cleaning takes place after the corresponding parts of the objective have been replaced.
Accordingly, the proposed solution was to avoid contamination of replaceable objective parts by preventing the parts for installation into the objective from being exposed to the ambient atmosphere after production and cleaning. This means, however, that the entire transport chain from storage to transport right into the objective must be carried out in a correspondingly clean atmosphere, a fact which represents considerable outlay. Moreover, there is a risk that, in storage, tiny amounts of contaminants in the sealed atmosphere around the replaceable objective part will in turn lead to deposits.